The 3rd Generation Partnership Project (3GPP) developed the standards used by Long Term Evolution (LTE) cellular communication systems. LTE is a standard for wireless data communications technology that has evolved from the GSM/UMTS standards. The goal of LTE is to increase the capacity and speed of wireless data networks using new Digital Signal Processing (DSP) techniques and modulation schemes that were developed in the past decade. The LTE wireless interface is incompatible with second generation (2G) and third generation (3G) networks, and operates via a separate wireless spectrum.
In LTE the purpose of the Radio Link Monitoring (RLM) function implemented in the mobile subscriber's user equipment (UE) is to monitor the downlink radio link quality of the serving cell in RRC_CONNECTED state and is based on cell specific reference signals (RSs). This in turn enables the UE, when in the RRC_CONNECTED state, to determine whether it is in-sync or out-of-sync with respect to its serving cell.
After a certain number of consecutive out-of-sync indications, the UE starts a network configured radio link failure timer. The timer is stopped if a number of consecutive in-sync indications are reported by the UE's physical layer. Both the out-of-sync and in-sync timers are configurable by the network. If the out-of-sync timer expires, Radio Link Failure (RLF) is declared. The UE turns off its transmitter to avoid interference and save power, and is also required to re-establish the RRC connection within a certain time.
Control information is provided by the base-station to the UE through the Physical Downlink Control Channel (PDCCH). The PDCCH can be used by the base-station to indicate to the UE that information needs to be received in the downlink. This channel can also be used to send uplink grant messages to signal to the UE that it is allowed to transmit. More generally, the PDCCH is used by the base station to convey scheduling decisions to the UE and hence correct reception and decoding by the UE of the associated control messages is critical to the correct operation of the network. The criteria for indicating out-of-sync and in-sync conditions are therefore based on whether the UE can reliably decode the PDCCH or not.
A number of different formats can be used for the transmission of the control information messages on the PDCCH in order to adapt to variations in the quality of the transmission link between the base-station and the UE. These different formats correspond to different levels of error correction redundancy and hence have different signal to noise ratio (SNR) requirements. An out-of-sync indication will be generated when a hypothetical PDCCH transmitted with a format using a high level of redundancy cannot be received reliably by the UE. On the other hand, if the UE estimates that it can correctly receive control messages transmitted on the PDCCH with a format using a low level of error correction redundancy, an in-sync indication will be generated. Hence, the generation of in-sync and out-of-sync indications requires the UE to estimate the Block Error Rate (BLER) for these two hypothetical formats.
The existing BLER estimation solutions are based on calculating the effective SNR over the complete bandwidth on a per subcarrier basis. The SNR is generated from the channel estimation process performed on the Reference Signals (RSs) transmitted by the base station. The ensemble of SNRs per subcarrier are mapped to an overall effective SNR using estimation methods such as mean mutual information per bit (MMIB) or exponential effective SNR mapping (EESM). This effective SNR is then used to estimate the BLER, based on the BLER versus SNR mapping function for the assumed PDCCH transport format (see for example 3GPP R4-081998 and 3GPP R4-082302).
The operating points for the detection of out-of-sync and in-sync correspond to cases of very low signal-to-noise conditions that are very challenging for channel estimation and the high noise level in the received signal has a major impact on the accuracy of the calculated SNR per subcarrier. The channel estimation noise in these cases is high and adds to the average channel power leading to a bias in the calculated SNR. The channel estimation noise and the resulting channel power bias make the effective SNR highly unreliable when the number of available measurements is limited, for example when Discontinuous Reception (DRX) is used or in Time Division Duplex (TDD) mode when the number of downlink subframes is small. The invention described herein provides techniques which improve the accuracy of the SNR estimation process and reduce the complexity of the radio link monitoring processing.